The Solomon’s Knot, or Swastika Pelta is an ancient symbol, and is commonly seen in Roman mosaics.
The motif is found in formal medieval church decoration. As a graffito it appears to be rare, although there is a particular concentration in part of East Anglia.
Its apotropaic function is based on the endless knot or line principal, by which a demon would be become fascinated by the line and follow it, thereby becoming trapped within the device.